This invention pertains to the separation of the ortho, meta and para isomers of ethyltoluene.
The industrial utility of ethyltoluenes is well-known. Pure p-ethyltoluene, as well as mixtures of para and meta ethyltoluene, are precursors to the corresponding vinyltoluenes, which are useful as cross-linking agents in coating applications. o-Ethyltoluene is a useful starting material in preparing intermediates for chemical and pharmaceutical syntheses.
Typically, dialkyl-substituted, monocyclic aromatic hydrocarbons, such as ethyltoluene, are prepared by direct alkylation of benzene which yields a mixture of the three familiar isomers: ortho, meta, and para. Separation of these isomers is complicated by the fact that they typically have similar melting and boiling points. For example, the boiling points of ortho, meta, and para ethyltoluene are 164.degree.-165.degree. C., 158.degree.-159.degree. C., and 162.degree. C., respectively. Thus, a distillation column designed to separate a mixture of these isomers is required to have a large number of plates, and even then the separation is inefficient. The fractional distillation of ortho, meta and para ethyltoluenes, for example, yields ortho-para and meta-para mixtures.
It is known in the art to separate the isomers of dialkyl-substituted, monocyclic aromatic hydrocarbons having eight or nine carbon atoms by the use of zeolite adsorbents. U.S. Pat. No. 3,699,182 discloses the use of aluminosilicate ZSM-5 and ZSM-8 zeolites for the separation of a mixture of para xylene from a mixture of the same with ortho xylene and/or meta xylene. It is also illustrated that mixtures of 2- and 4-ethyltoluenes can be separated. Disadvantageously, these adsorbents do not separate all three isomers of the above-identified C.sub.8 and C.sub.9 aromatic hydrocarbons.
U.S. Pat. No. 4,376,226 teaches the use of crystalline aluminosilicate adsorbent CSZ-1 for the separation of a C.sub.8, C.sub.9 or C.sub.10 aromatic feedstream. The adsorbent is applicable to a mixture of ethyltoluenes: however, the separation is not particularly effective. As a further disadvantage, the adsorbent contains a high concentration of aluminum which introduce acidic sites which may catalyze undesirable side reactions, such as isomerization and cracking.
U.S. Pat. No. 4,482,777 discloses a process for separating the ortho-isomer from a feed mixture comprising at least two bialkyl substituted monocyclic aromatic isomers of C.sub.8 to C.sub.18 carbon atoms, including the orthoisomer. The adsorbent is a crystalline aluminosilicate zeolite of the X or Y class. Disadvantageously, this process is not particularly efficient, since mixtures of the ortho and para isomers and the ortho and meta isomers are obtained. Even more disadvantageously, zeolite X has limited thermal stability. As a further disadvantage, the adsorbent contains a high concentration of aluminum which introduce acidic sites which may atalyze undesirable side reactions, such as isomerization and cracking.
Other aluminosilicate adsorbents are known for the separation of the above-identified alkylaromatic hydrocarbons, including zeolite beta as described in U.S. Pat. No. 4,554,398. Despite the advances made in the art, however, a need exists for the improvement of such isomer separations. What is needed is a simple process of separating ethyltoluene isomers which does not require expensive and complicated engineering, such as large distillation columns. What is also needed is a clean separation process which does not produce undesirable by-products. Most needed, however, is a simple and clean separation process having a high degree of efficiency such that essentially pure ortho, meta and para ethyltoluene isomers are obtained.